In general, a thermal recording material (thermal recording medium) is obtained by pulverizing and dispersing a basic dye which is colorless or pale at ambient temperature and an organic developer as ultrafine particles, mixing them, mixing them with a binder, a filler, a sensitizer, a lubricant, other additive and the like, applying the resulting coating solution to a support such as paper, film plastic and the like, and drying to form a heat-coloring layer, wherein the heat-coloring layer of the thermal recording material is heated with a thermal head, a thermal pen, a laser beam and the like to afford developed color records. The basic principle of this recording method is considered to be based on the chemical contact of an electron donative dye and the organic developer in the heat-coloring layer, which changes the dye to a chromoplast. Since this recording method (i.e., thermal recording method) is characterized in that it does not require complicated treatments such as development, fixing and the like, as compared to other recording techniques (recording methods) now in practice, recording can be done in a short time at a comparatively low cost with a simple recording apparatus (recording means), maintenance is not necessary, noise does not occur during recording, and the obtained color is highly clear, it has been widely used as a recording method for printers for computer output, calculator and the like, recorder for medical measurement, facsimile, automatic ticket machine, labeling field, copying machine and the like. In recent years, however, the required quality of thermal recording materials (thermal recording media) is becoming higher as versatile and high performance apparatuses of those mentioned above have been developed and, for example, since the thermal energy of thermal heads of recording apparatuses tends to become very small as high speed recording and miniaturization of apparatuses proceeds, thermal recording materials (thermal recording media) are requested to have sensitivity enabling formation of high density and clear color image even with a small thermal energy.
As developer to be contained in the heat-coloring layer of thermal recording materials, various compounds having a phenolic hydroxyl group have been conventionally proposed (e.g., JP-B-40-9309, JP-B-43-4160, JP-B-45-14039, JP-B-51-29830, JP-A-56-144193 etc.). Of these, bisphenol compound, 4-hydroxybenzoate and the like have been practically used alone or in combination of several kinds thereof. However, thermal recording materials formed using such conventionally proposed developers have many problems of, for example, low heat response, insufficient color density obtained by high speed recording, uneven colors, time course changes in color image density after recording, discoloration during preservation, degraded heat-stable color of non-image areas, white powdery surface precipitation (so-called blooming), low reprintability and the like.
Hence, JP-A-9-278695, JP-A-2001-96926 and the like recently disclose methods using trisphenol compounds as developer or color degradation inhibitor. According to the study of the present inventors, however, addition of these compounds to the heat-coloring layer failed to afford sufficiently high color sensitivity. In addition, as developers, JP-A-58-181686 discloses a method comprising use of a 2,2′-methylenediphenol compound, WO02/098674 discloses a method comprising use of a condensate of a 2,2′-methylenedi-t-butylphenol compound and a condensed composition thereof, and WO03/029017 discloses a method comprising use of a condensate of a 2,2′-diphenol compound and a condensed composition thereof. However, use of the phenolic compounds described in these publications as developers failed to afford sufficiently high color sensitivity, and image stability such as heat resistance, moisture resistance, weatherability and the like was not sufficient. Moreover, JP-A-2003-154760 and JP-A-2003-154761 tried to improve image stability by a combined use of the aforementioned phenolic compounds (developer) with a stabilizer and a particular sensitizer, but these methods also failed to afford sufficient color sensitivity to meet the recent high-speed recordation and low energy consumption. As the situation stands, a thermal recording material sufficiently satisfying the recent requirement for high sensitivity as well as good image stability has not been obtained yet.
In view of the above-mentioned situation, the problem of the present invention is to provide a novel embodiment of a developer enabling to afford a thermal recording material sufficiently satisfying the recent requirement for high sensitivity, and good preservation stability of color image and non-image areas (particularly heat resistance and moisture resistance) and a thermal recording material using the developer, which has good preservation stability (particularly heat resistance and moisture resistance) of color images and non-image areas.